When forming thru-holes in an electronic lamination substrate such as a multilayer printed circuit board panel, metallic foil is usually placed on the above circuit board panel (sometimes referred to as the panel hereinafter) and then thru-holes are formed with a gimlet, a drill, a puncher or the like. There are problems that resultant debris sticks to the panel surface, the thru-hole walls or the edges surrounding the thru-hole during the method of forming the thru-hoes. Furthermore, there is another problem that the thru-hole wall formed with a rough surface. Therefore, such debris must be forcefully removed by installing a suction device additionally, or debris stuck to a neighboring portion of the formed thru-holes must be removed by grinding cut surface in conventional methods. However, it is not preferable to add these processes like sucking debris or grinding, which make the working process complicated.
To simplify such a process, there are methods that a sheet impregnated with a water soluble lubricant is placed on one side or both sides of the above lamination substrate, in which metallic foil is layered on an insulating material, and then thru-holes are drilled through this sheet by a drill or the like, in the lamination substrate U.S. Pat. No. 4,781,495 and U.S. Pat. No. 4,929,370). That is, the mixture, solid water soluble lubricant, comprising glycols such as diethylene glycol or dipropylene glycol, ester synthetic wax such as fatty acid, and a non-ionic surfactant is impregnated into a sheet such as paper, which is employed for drilling in the above method. Further, besides this method, there are methods of drilling thru-holes in a lamination substrate through a sheet wherein a high molecular layer is formed with water soluble high molecular polyethylene glycol or polypropylene glycol solely, or water soluble high molecular mixture thereof, or fatty acid ester thereof, or the like (Tokkaihei 4-92488, Tokkaihei 4-92489, Tokkaihei 4-92490, Tokkaihei 4- 92491, Tokkaihei 4-92492, Tokkaihei 4-92493, and Tokkaihei 4-92494 ).
However, the above methods have various defects. For example, when forming thru-holes, the heat removal effect on a drill bit is not sufficient, stickiness occurs, lubricating effect cannot be obtained because the water soluble high molecular compound is too hard to melt, or the water soluble high molecular compound cannot be easily formed into a sheet due to inferior film formability which stems from poor thermoplasticity.
Furthermore, when using a metallic foil on which the water soluble compound layer is formed or a wear plate of metallic foil wherein the water soluble compound layer is directly formed therein to form thru-holes by a drill or a gimlet, the above water soluble compound may sometimes stick to the drill or the gimlet. For example, when forming thru-holes with a drill or a gimlet at a high speed rotation such as 50,000 to 100,000 rpm, since debris of the above water soluble compound sticks to the blade of the drill or the gimlet, the rotation balance may be disturbed. Therefore, the bit of the drill or the gimlet passes through the lamination substrate slantwise, not vertically. If a few lamination substrates are layered in such a forming process, the tip of the drill or gimlet passes through it slantwise, resulting in accuracy deterioration because the discrepancy is caused between the hole positions of the entry sheet and the exit sheet. For example, if the discrepancy between the entry sheet and the exit sheet reach 200 .mu.m, layer processing becomes impossible.
Especially, the diameter of thru-holes has recently become smaller such as 0.5 mm, 0.3 mm, 0.25 mm and 0.20 mm due to the rise of packing density. For this reason, it is required to effectively control frictional heat caused in cutting with the drill and also heighten accuracy of the positioning of the thru-holes.